There has lately been an increasing demand for still higher strength of steel products from the viewpoint of enhancement in fuel economy attendant on reduction in weight of the automobile and so forth, and also from the viewpoint of reduction in exhaust gas, and in particular, with respect to a cold rolled steel sheet, rapid progress toward higher tension (higher strength) has since been made. On the other hand, however, there is not a small demand for high strength and thick steel products for the purpose of enhancement in rigidity from the viewpoint of safety upon collision. With the cold rolled steel sheet, however, it is difficult to cope with such a demand in terms of production facilities, and cost, so that it becomes necessary to cope with such a demand with a hot rolled steel sheet.
Even in the case of making use of the hot rolled steel sheet, press working is required to work the hot rolled steel sheet into a product shape, as with the case of the cold rolled steel sheet, so that ductility such as elongation, and so forth cannot be slighted even though higher strength is aimed at. However, not only the hot rolled steel sheet but also material for steel products, in general, has a natural tendency that the higher the strength, the lower ductility becomes, thereby causing deterioration in workability. Accordingly, with respect to the steel products, addition of an alloying element capable of enhancing strength without causing deterioration in ductility has been under study.
Mo, among others, has attracted attention as an element useful for enhancement in strength without causing much deterioration in ductility. In addition, Mo promotes formation of a bainite structure contributing to an increase in strength by checking formation of a ferrite structure, occurring in a cooling process after completion of hot rolling, and eliminates the need for such process control as to raise a heating temperature of a slab before hot rolling, and to adopt low coiling temperature, so that attention is focused on Mo as an alloying element to be added to produce a high strength hot rolled steel sheet.
Notwithstanding the above, if Mo as the alloying element is added, this will cause deterioration in phosphatability, which leads to poor adhesion of a coating film after electrodeposition coating, thereby causing a problem in that a finished product is adversely affected in respect of external appearance and corrosion resistance, and so forth.
Meanwhile, there have since been proposed several methods for improving a surface condition (for example, microscopic asperity pattern) of an in-process steel sheet in order to enhance the phosphatability of a steel sheet.
For example, Patent Document 1 has disclosed a hot rolled pickled steel sheet with phosphatability enhanced by specifying a microscopic shape of a surface of a steel sheet. This technology is for adjusting a surface condition of the steel sheet by subjecting the steel sheet to skin-pass rolling with the use of a roll (dull roll), provided with an asperity pattern formed on the surface thereof by use of high-energy beams, thereby transferring the asperity pattern on the surface of the roll to the surface of the steel sheet. With this method, however, an increase in cost, due to addition of process steps, such as working of the roll into the dull roll, and the skin-pass rolling, is unavoidable, and furthermore, with respect to a Mo-added steel as an object of the invention, satisfactory effects have not been obtained.
Further, Patent Document 2 has disclosed a method for enhancing phosphatability by controlling an average grain size of a high tensile hot rolled steel sheet with Ti added thereto at not more than 3.0 μm, and controlling surface roughness (Ra) thereof at not more than 1.5 μm. With this method, however, with respect to the Mo-added steel, intended effects have not been obtained either.
Still further, Patent Document 3 has disclosed a technology for controlling a microscopic asperity pattern on the surface of a steel sheet. This technology, however, is intended to improve coating sharpness and press workability through controlling respective diameters of projections in the microscopic asperity pattern to fall in a range of from 50 to 200 μm, which is by far larger than several micrometers representing a grain size of zinc phosphate on which the present invention focuses attention as a determinant affecting the phosphatability. Hence, the technology hardly contributes to enhancement in the phosphatability    [Patent Document 1] JP-A No. 187202/1990    [Patent Document 2] JP-A No. 226944/2002    [Patent Document 3] JP-A No. 293503/1993